What is the no-go area around a crossed pair of figure-of-eight microphones?

A crossed pair of figure-of-eight microphones is sensitive
to the front, sensitive to the rear, but elsewhere there is a 'no go'
area? Why is this?

It is common to record in stereo using a coincident crossed pair of figure-of-eight microphones.

'Coincident'
means that the diaphragms of the mics are as close together as
possible. 'Crossed' means that one mic points left, the other points
right, separated by 90 degrees.

'Pair' means there are two of them :-)

A coincident crossed pair always works, if you can find the right position for the mics, and captures a good clear recording.

When
figure-of-eight pattern mics are used for the crossed pair there is an
interesting feature - since the mics are equally sensitive to the front
and to the rear, the rear is effectively another 'front'.

This is
commonly exploited in drama recording where a group of actors can divide
themselves between front and rear, and in the resulting recording they
will all seem to be located together.

However there are two 'no-go' areas around a crossed pair of figure-of-eights.

Sound
that approaches the mics from the sides has the unfortunate effect of
striking the front of the diaphragm of one mic, and the rear of the
diaphragm of the other. So the two diaphragms are pushed and pulled in
opposite directions.

In other words, the signals are out of phase, or inverted, with respect to each other.

When
this is played through loudspeakers or headphones, the net result will
be one eardrum being pushed inwards while the other is sucked outwards,
and vice versa.

There is no sound in nature that does this, so the
human hearing system has difficulty to interpret what it is hearing. It
sounds distinctly odd.

So the moral of this story is never to
place sound sources in either of the two quadrants to the sides of a
crossed pair of figure-of-eight mics.

This is fine for direct
sound sources where you have control. But you have little control over
reflections reaching the sides of the pair.

Out of phase reverb is
a characteristic of crossed figure-of-eights and perhaps accounts for
the fact that although in theory this configuration should produce
perfect stereo sound, in practice it doesn't quite live up to that
ideal.